Fuel Filler Cap
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2003 Mercury Sable, 2003 Mercury Mountaineer, 2003 Mercury Marauder, 2003 Mercury Grand Marquis, and 2003 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.
The fuel filler cap is used to prevent fuel spill and close the evaporative emission/fuel system to atmosphere. Some vehicles may have a Fuel Cap Off Indicator Lamp (FCIL) in the instrument cluster which will illuminate when there is a failure in the vapor management system that may be due to the fuel filler cap not being sealed.